1. Field of the Invention
The present invention relates to the determining of a horizontal path that an aircraft has to follow when going from one obligatory point of passage to another, in keeping with the constraints dictated by the instructions for the turning radius and the instructions for the direction of approach in terms of route or course to be maintained at the obligatory points of passage.
2. Discussion of the Background
Hitherto, the horizontal path to be followed during a mission has been constructed on the basis of a broken line passing through obligatory points of passage, with turning portions between the rectilinear path segments travelled in a constant route or course. This broken line is not chosen haphazardly but on the basis of standard criteria of optimization of the length of the trace on the ground and the maneuvering possibilities of the aircraft. At present, this choice and therefore the construction of the horizontal path to be followed during a mission require major computation means that are as yet not available on aircraft. The horizontal path to be followed is therefore computed by means located on the ground when a mission is being prepared.
Once defined on the ground, all the information and characteristics pertaining to the mission, namely the sequence of rectilinear segments and turning portions that constitute the mission as well as the instructed speeds dictated for traveling in each of the rectilinear segments and turning portions, are put into shape so that they can be used by the flight management system of the aircraft. A rectilinear segment is defined by the geographical coordinates of its extremities and possibly by a direction of orientation in route or course while the turning portions are defined by a center and a radius of curvature.
To track the horizontal instructed path, the flight management system of an aircraft constantly compares the true geographic position of the aircraft, as it appears from the localizing measurements obtained through the processing of the information from the inertial guidance unit and/or localizing instruments such as satellite localizing systems or the like, with the geographical position that it is supposed to have on the instructed horizontal path. From these comparisons, it deduces a lateral divergence and a route divergence and, by means of a table of equivalence with two inputs, lateral deviation and route deviation, it converts these two deviations into commands applied to the rudders by means of the flight control system of the aircraft.
This method has drawbacks, inasmuch as during a mission it is not unusual for an aircraft to have to modify its flight plan following unexpected events, for example malfunctioning or the appearance of a danger zone on the path being followed. The new path to be followed, which is different from the one initially planned, has neither been prepared beforehand nor put into shape for processing by the flight management system of the aircraft which then cannot be used. The pilot must therefore resume control of the flight commands and himself follow a new path that resolves the problem resulting from the appearance of the event.
To release the pilot from the piloting task, even in unexpected situations, it has already been proposed to equip aircraft with an additional computer capable of suggesting new paths with which to resolve the problem raised. However, such equipment proves to be very costly and does not entirely meet the pilot""s needs because it requires a major response time that is often incompatible with the reaction times that an aircraft pilot must have. Furthermore, the paths proposed are often ill-suited to the problem that has arisen. They, lack precision and cannot be directly performed by the flight management system. The pilot himself has to take over the flight controls and follow the instructions given to him on the proposed path.
Furthermore, resuming the normal route of the mission implies that the pilot should return to the initially planned instructed path without the help of the flight management system which is capable of making automatic corrections of only small positional deviations.
The modifications that are made during a mission, when there is an unplanned change in route, can vary to a great extent in one and the same situation as they are left entirely to a pilot""s initiative and do not take a form that can be directly exploited by the aircraft navigation system. These modifications adversely affect the planning precision of the mission because they cause advances or delays which are difficult for the pilot to assess with precision. The pilot can then only compensate for them imperfectly. Furthermore, the high possibility of unplanned modifications in the path followed during a mission means that it is necessary to allow for a substantial safety margin in terms of the quantity of fuel taken on board. This will be done to the detriment of the carrying capacity of the aircraft.
It is the aim of the present invention to overcome these drawbacks.
The invention is also designed to facilitate the pilot""s task by releasing him from the problems of navigation related to unexpected changes in route and by transferring these responsibilities to the flight management system of the aircraft. To this end, there is proposed a routing system, undemanding in terms of computation capacity, that can be executed by the flight management system of the aircraft whenever there is a change in route to update the path to be followed and immediately integrate the modifications made in the path into the flight plan so that the navigation system can take account thereof in real time and maintain the scheduling precision of the mission by itself, after homing in on the instructed path, by playing on the speed of the aircraft in the period that precedes this homing in.
An object of the invention is a method for the horizontal routing of an aircraft between two obligatory points of passage in keeping with the constraints dictated by the instructed values of turning radius and by the instructed values of route or course orientation to be followed at the obligatory points of passage.
According to the invention, this method consists in:
determining, at the obligatory starting point of passage, called the initial point, the turning paths that meet the instructed value of turning radius at this initial point, these turning paths being located on two circles known as homing circles, tangential to the initial point along the direction of the instructed value of orientation in route or course at this initial point, known as the directional sense of the initial route, and oriented along this direction therefore with reverse directional senses of travel,
determining, at the obligatory arrival point of passage, called a final point, the turning paths that meet the instructed value of turning radius at this final point, these turning paths being located on two circles, called capture circles, tangential to the final point along the direction of the instructed value of orientation in final route or course, called the directional sense of the final route, and oriented along this direction, therefore with two reverse directional senses of travel, and
defining a path comprising three portions:
a first portion consisting of an initial circular turn for homing in on the general direction of the final point, begun in starting from the initial point with the direction of the instructed value of orientation of an initial route and followed in describing a part of the contour of one of the homing circles in following its directional sense of travel,
a second portion consisting of a constant-route homing straight line along a tangent to the homing circle selected for the initial circular turn and to one of the capture circles, said tangent being oriented towards the final point and in the directional sense of each of the circles at the points of tangency, and
a third portion consisting of a final capture circular turn following the contour of the capture circle reached at the end of the homing straight line in keeping to its directional sense of travel, until the final point reached with the direction of the instructed value of orientation of final route,
the homing and capture circles followed during the homing and capture circular turns being selected so as to minimize the distance of travel, and
making the aircraft follow the path thus defined.
Through these arrangements, the desired path is obtained with a minimum number of computations, given all the possible situations of positions of initial and final points and route points. This method furthermore makes it possible to select the shortest path before computing it. The result thereof is that this method can easily be applied to the determining of the path in real time. The path thus computed is obtained by taking account of the maneuvering possibilities of the aircraft. It therefore enables the precise overflight of the initial and final points with the planned courses.
Generally, automatic piloting devices are designed to perform straight line phases and constant radius turns, the path obtained by this method being therefore quite suitable for being fed into a device of this kind.
According to one particular feature of the invention, the method applies the convention according to which the first capture circle is the one located on the same side of the final route going through the final point as the initial point and the first rallying circle is the one which has the same directional sense of travel as the first capture circle. Through these conventions, the capture circle C1 will be selected in most cases.
The method includes the selection of the first homing circle if the initial point is in the first capture circle and the selection of the first capture circle if the first homing circle does not intercept the final route passing through the final point or intercepts the second capture circle, and the selection of the second capture circle if the first homing circle intercepts the final route but not the second capture circle.
The method according to the invention thus makes it possible to meet the conditions of route and point of passage even if the final route is very close to the initial point, given the maneuvering possibilities of the aircraft.
It must be noted that, in this case, the homing and capture circles are selected by making use of simple geometrical computations that cost little in computation time.
According to another particular feature of the invention, the method includes the a priori selection of the first capture circle if the initial point is not in the first capture circle.
According to another particular feature of the invention, the method includes the selection of the first homing circle and the first capture circle if the following condition is met:
the initial point is not in the first capture circle, and
the second homing circle intercepts the first capture circle.
Should the initial point not be in the first capture circle and should the second homing circle not intercept the first capture circle, the homing circle is selected as a function of the initial route and of the route directed along the tangent to the first capture circle, going through the initial point.
According to another particular feature of the invention, if the following condition is fulfilled:
the initial point is upline with respect to the final point and outside the first capture circle, and
the first homing circle intercepts the final route but not the second capture circle, the method comprises the selection of the first homing circle and the second capture circle or of the second homing circle and the first capture circle.
In this case, one combination or the other is selected depending on whether the first capture circle has been previously selected or not.
In this way, using tests that cost little in terms of computation capacity, the method according to the invention is used to select the homing circle and the capture circle to be used to determine the desired path. It furthermore makes it possible to limit the cases where costlier computations of tangents have to be performed.